A Survey of z~6 Quasars in the SDSS Deep Stripe. II. Discovery of Six Quasars at z_{AB}>21

TL;DR

This paper reports the discovery of six new faint quasars at redshift around 6 from the SDSS deep stripe, providing insights into the quasar luminosity function and black hole growth in the early universe.

Contribution

It presents the discovery of six new faint z~6 quasars and characterizes the quasar luminosity function at this epoch, extending previous surveys to lower luminosities.

Findings

The quasar luminosity function follows a power law with slope -2.6 to -3.1.

Approximately 40% of these quasars have narrow Lya emission lines.

The quasars suggest rapid black hole growth and high Eddington ratios.

Abstract

We present the discovery of six new quasars at z~6 selected from the Sloan Digital Sky Survey (SDSS) southern survey, a deep imaging survey obtained by repeatedly scanning a stripe along the celestial equator. The six quasars are about two magnitudes fainter than the luminous z~6 quasars found in the SDSS main survey and one magnitude fainter than the quasars reported in Paper I (Jiang et al. 2008). Four of them comprise a complete flux-limited sample at 21<z_AB<21.8 over an effective area of 195 deg^2. The other two quasars are fainter than z_AB=22 and are not part of the complete sample. The quasar luminosity function at z~6 is well described as a single power law \Phi(L_{1450}) \propto L_{1450}^{\beta} over the luminosity range -28<M_{1450}<-25. The best-fitting slope \beta varies from -2.6 to -3.1, depending on the quasar samples used, with a statistical error of 0.3-0.4. About 40% of the quasars discovered in the SDSS southern survey have very narrow Lya emission lines, which may indicate small black hole masses and high Eddington luminosity ratios, and therefore short black hole growth time scales for these faint quasars at early epochs.